There are a number of commercial sewing machines on the market, one such sewing machine being described in U.S. Pat. No. 4,825,781 issued May 2, 1989. Machines of this type do a very good job of sewing together two pieces of material, such as the seam of a pants leg; however, such machines do have certain limitations.
The first such limitation is that the machine operator cannot load new material to be sewn into the machine until sewing on the prior material is completed. The loading operation is also completely manual. Thus, for one exemplary machine, while it takes approximately six seconds for the machine to sew a pants leg, it can take up to twelve additional seconds for a new piece of material to be loaded into the machine. It thus takes approximately eighteen seconds to sew a pants leg in this machine, with only approximately one third of that time being used for actual sewing. Therefore, in order to increase production from a given machine, it is desirable to be able to reduce the time required for loading the machine, and in particular to permit preloading of material into the machine so that there is some overlap between loading time and sewing time. By simplifying the load procedure permitting preload so that the loading operation may occur at the same time the sewing operation is being performed, and permitting the actual loading operation to be performed automatically, it should be possible to reduce the time required to sew for example a pants leg by as much as 50%, and even greater time savings may be possible.
The second problem is that, no matter how carefully the material is originally cut, there are frequently differences in the length of the two pieces of material being sewn. It is known that such differences in length can be compensated for by changing the feed rate at the sewing head for either one or both of the pieces of material so that on of the pieces of material has a little extra fullness in some area and/or the other piece of material is stretched slightly. For a pants leg, it is preferable that this adjustment be made over the top third of the leg (i.e. in the area above the knee). However, heretofore, the operator has been relied upon to estimate the amount of mismatch between the two pieces of material being sewn together and to manually adjust the feed rate to compensate for such mismatch. This extra labor on the part of the operator accounts in part for the time required to load ne pieces of material to be sewn into the machine. The need for the operator to estimate and make manual adjustment for mismatches also means that the operator must have a certain skill level, the need for more highly skilled operators also increasing costs. Thus, production time could be reduced, thus further reducing cost, if a mechanism were provided for automatically detecting mismatches in the pieces of material to be sewn and compensating for such mismatches.
A need therefore exists for an improved method and apparatus which increases the production obtainable from a sewing machine, and thus reduces the cost of operation thereof, by permitting preloading of material to be sewn during a prior sewing operation, and by permitting automatic detection of material mismatches and compensation for such mismatches.